This invention relates to a self-tensioning piston ring which, in the closed (circumferentially compressed) state, has a non-uniform, radially outwardly oriented pressure distribution.
In piston rings, apart from the shape of the cross section and the running face of the piston ring as well as the properties of the ring material, the radial pressure distribution has also a decisive effect on the favorable wear conditions, particularly of the running face of the piston ring. High performance piston rings essentially have-viewed about the circumference of the ring-a non-uniform radial pressure distribution; the manufacturers of piston rings seek a pressure distribution which corresponds either to a so-called four-stroke contact characteristic or a two-stroke contact characteristic.
In piston rings having a four-stroke contact characteristic, the radial pressure is higher at the ring gap zone (that is, along a ring length portion on either side of the ring gap) to thus generate a restraining force at the gap to reduce the full effect of ring flutter at the gap.
Piston rings having a two-stroke contact characteristic have in the ring gap zone a radial pressure which is reduced relative to piston rings of four-stroke contact characteristic. The higher radial pressure is relocated into the quadrants separated by the gap. Such piston rings are discussed in an article by A. Mierbach et al, entitled "Heat Flow Through Piston Rings and its Influence on Shape" published by the Society of Automotive Engineers, Inc. in the SAE Technical Paper Series No. 831283, September 1983. This article discloses that a significant portion of the heat flows from the piston to the cylinder across the piston rings. As a result, the piston ring is, at the inner diameter, hotter than at the outer diameter where the ring contacts the cylinder wall. This fact affects the sealing behavior of the piston ring both during engine start (cold engine) and during operation (warm engine).
In the cold state, a piston ring of four-stroke contact characteristic engages the cylinder wall with a clearance in a light-tight manner. During operation (warm engine), the radius of curvature of the piston ring increases compared to the cold condition, because, due to the higher temperatures, at the inner diameter the ring expands to a greater extent and thus stretches more than at its outer surface. Such a change in the radius of curvature leads at the gap zones to a non-sealing contact characterized by forces concentrated in points. The piston ring which is then no longer light-tight is exposed to excessive running face wear at the gap zones and has a high oil consumption during the break-in period.
A piston ring having a two-stroke contact characteristic behaves precisely in an opposite manner compared to a piston ring of four-stroke contact characteristic. In the cold condition the ring engages the cylinder wall at the gap zones in a manner which is not light-tight, whereby cold starting problems arise. By virtue of the thermal expansion, the gap zone of the piston ring engages the cylinder wall only during the operational phase (warm engine).